There are many processes wherein protic acids are generated. In most of these processes it is desirable to recover the protic acids. In certain processes the protic acid is part of a waste stream and failure to recover the acid from the waste stream results in environmental damage. In other processes, the acid is a valuable product which can be used for further chemical processing, or can be recycled in the process. Thus, there is a need to recover such protic acids from waste streams or from reaction media.
A number of methods have been developed to recover such protic acids. One common method is to contact the acid with a basic compound such as an alkaline earth metal base, an alkali metal base, or an amine to form a salt. See, e.g., Morrison and Boyd, Organic Chemistry, 3rd ed. (1975) 731. There are several problems with this kind of process. First, in some of these processes, it is very hard to effect complete recovery of the acid from the system, since the salt formed may be hard to recover, or there may be acid that does not react with the base. When alkali metal or alkaline earth metal bases are used, the halogen value cannot be recovered in usable form. When an amine is used to recover the acid the amine may decompose when one tries to recover the acid from the base. In such cases it has been found that the dissociation conditions are harsher than the conditions at which the amine used to recover the acid undergoes decomposition. In either case, the acid is lost where the salt is lost, or the amine undergoes decomposition, either before the acid can be recovered or before the amine can be recycled.
Another method of recovering acids and organic bases from their mixtures is disclosed in U.S. Pat. No. 3,651,166. That patent discloses the use of organic bases, such as pyridine, the N-alkyl pyrrolidones, acetamide and dimethylformamide, to form organic baseacid salts or complexes, then dissociating these salts at a temperature lower than that at which decomposition of either, or both, the acid or the organic material takes place. In the case of pyridine, recovery of the pyridine is only possible when the salts are formed with weak organic acids, such as benzoic, naphthoic, and the like, and not with strong protic acids. An example of a typical process employing an organic base wherein the base is converted to an acid salt is described in U.S. Pat. No. 3,360,583. That process relates to the production of polyolefinic materials by reacting a haloether with an olefin to form a haloether adduct of the olefin and splitting alcohol hydrogen halide from the intermediate adduct in the presence of an organic base to form a conjugated polyolefin and the hydrogen halide salt of the organic base. More specifically, chloromethyl methyl ether is reacted with isobutylene to form an intermediate adduct. The adduct is then heated in the presence of N-methyl-2-pyrrolidone to convert the adduct to isoprene. The N-methyl-2-pyrrolidone is simultaneously converted to its hydrogen halide salt. Thus, these two patents take together provide a process for formation of salts and their dissociation. However, one problem is encountered. The pyrrolidone will not combine with strong protic acids to form salts. Thus, the process of U.S. Pat. No. 3,651,166 cannot be used to recover these protic acids.
Even where recovery of protic acids can be accomplished using any of the known methods, these methods of recovery do not distinguish between differing acids present in one stream. In many instances it is necessary to deal with mixtures of protic acids, rather than pure acids, because a number of chemical processes result in, or produce as byproduct, a mixture of HCl and HBr. For example, a mixture of these acids can be produced in the manufacture of HCl. See, e.g., Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd ed., vol. 11 (1967) 307-337, particularly 314-323.
It is often desirable in the art to separate HCl and HBr to allow recovery of either or both as pure compounds. A commonly used method of separation involves selective absorption of the given gas. For example, U.S. Pat. No. 3,488,923 shows the solvent extraction of hydrogen halides. The mixture of hydrogen chloride and hydrogen bromide is contacted with a solvent system comprising a phenolic compound and a polyether with or without zinc chloride or zinc bromide. U.S. Pat. No. 4,036,939 shows a method of recovering hydrogen chloride from an organic solvent loaded with HCl in a dilute aqueous solution by carrying out, prior to distillation, a dehydration of the loaded organic solvent in a dehydration extractor in which the solvent is circulated countercurrent to an aqueous solution having a high chloride solution.
Finally, U.S. Pat. No. 4,254,092 discloses a method of separating HCl from a mixture of HBr and HCl using the difference in the stability to heat of the amine hydrohalides thereof. A mixture of HBr/HCl is treated with a solution of a tertiary amine and the HCl is split off, by the action of heat, from the amine hydrochlorides in the resulting mixture of amine hydrochlorides and amine hydrobromides. The HBr is recovered from the residual amine hydrobromides with the aid of a base. Suitable bases are, for example, sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide, sodium carbonate, and potassium carbonate. This process has two drawbacks, however. First, not all of the original amine is regenerated when the amine HCl salt is cleaved to recover the HCl. Thus, it is impossible to separate all of the HCl and recycle the amine. Second, this amine decomposition contaminates the final product.
Further information as to the physical properties and methods of preparation of hydrogen bromide and sodium bromide, prepared by the neutralization of sodium carbonate or hydroxide with hydrobromic acid, are disclosed in Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd ed., vol. 3 (1964) 767-768 and vol. 18 (1969) 484-485.
Thus, what is needed is a method of separating HCl and HBr from mixtures thereof, that does not suffer the drawbacks of the prior art and is economical and efficient.